1. Field of the Invention
This invention relates to a method of sintering a ceramic composition comprising oxides as at least one component.
2. Description of the Prior Art
Generally, ceramics consist of oxides only and sintering thereof is carried out in a gaseous atmosphere composed of at least one gas selected from oxygen, nitrogen, carbon dioxide, argon and helium gases at substantially normal pressure using a gas furnace. Of late, however, vacuum sintering, hot press or hot isostatic press has been employed for the purpose of obtaining fine particle and high density ceramic materials, and ceramic compositions have extended over a wide range such as including carbides in addition to oxides.
In the vacuum sintering of a ceramic composition comprising oxides substantially or as a part of the composition, a sintering furnace cannot help using carbon as a material of a heater or heat insulating felt from an economical point of view. In this case, the inside of the furnace is in a reducing atmosphere as shown by the following formulas and lower oxides with high vapor pressures are formed in a sintering at a high temperature: EQU H.sub.2 O+C.revreaction.H.sub.2 +CO (1) EQU 1/y A.sub.x O.sub.y +H.sub.2 .revreaction.(x/y)A+H.sub.2 O (2) EQU [1/(y-z)]A.sub.x O.sub.y +H.sub.2 .revreaction.[1/(y-z)]A.sub.x O.sub.z +H.sub.2 O (3)
In these formulas, A is a metallic atom such as Al, Zr, Si, Mg, Y, Ti, Cr, Ni and the like and x,y and z are respectively atomic ratios of a metal and oxygen, y being greater than z.
That is to say, water adsorbed in powdered materials or in the inner walls of the furnace reacts with carbon from a carbon heater, a carbon heat insulator or a combined carbon, or water reacts with the free carbon in carbides added to form carbon monoxide gas and hydrogen gas, and the oxides in a ceramic composition are reduced with this hydrogen gas to form free metals or lower oxides with high vapor pressures.
Through the above described reducing reactions, troublesome problems take place such that the carbon heater, carbon heat insulator, etc. are degraded to shorten their lives, the interior of the sintering furnace is remarkably contaminated, and lower oxides sublimate from the surface of the sintered body to degrade the surface roughness and to change the desired dimensions of the sintered body as well as to decrease the smoothness of the surface thereof. These problems can be solved to some extent by the use of a hot press, but the hot press has the disadvantage that it is very high priced and unsuitable for the production, in particular, the mass production of an article with a complicated shape.